1. Field of the Invention
The present invention generally relates to devices for optically splicing waveguides such as optical fibers, and more particularly to a vented, hinged splice element having improved hinge registration and clamping of the optical fiber.
2. Description of the Prior Art
Splices for optical fibers are known in the art. The most critical characteristic of an optical fiber splice is its insertion loss, i.e., the signal loss due to misalignment of the fibers, which may occur in three different manners. First of all, the fiber end faces should abut each other as closely as possible (end offset). The provision of a refractive index matching medium (gel) at the interface may mitigate the effects of any air space left between the end faces. Secondly, the fiber axes proximate the interface should be generally parallel, so that light exiting one fiber will strike the surface of the second fiber as closely as possible to a right angle, or 0.degree. angle of incidence (axial or angular offset). Finally, the axes of the fibers should be transversely aligned to maximize the overlapping surface area between the end faces (lateral or transverse offset). This alignment is critical since the diameter of the central glass core of single mode fibers is only about 8 .mu.m, so a deviation in axial alignment of as little as 1 .mu.m may result in a significant loss.
Several prior art optical fiber splicing devices attempt to optimize fiber alignment by utilizing a chip or tray having one or more grooves therein which receive the optical fibers. See, e.g., U.S. Pat. Nos. 3,864,018; 4,028,162; 4,046,454; 4,102,561; 4,220,397; 4,730,892; and 4,865,413. The grooves in the substrate provide a simple method for holding the fibers, which are forcibly held in the grooves by a compression plate or adjacent groove tray, or by the use of adhesives. The grooves may be concave or V-shaped. Concave grooves result in two primary points of contact with the fiber, while a V-groove with an opposing flat surface provides three points of contact. V-grooves in two opposing trays result in four points of contact, as shown in FIG. 4 of U.S. Pat. No. 4,046,454.
Some prior art splices combine the V-groove concept with a foldable or hinged splice element. See, e.g., U.S. Pat. Nos. 4,029,390; 4,254,865; 4,818,055; and 4,865,412; and Japanese Patent Applications (Kokai) Nos. 53-26142 and 58-158621. This basic design offers several advantages, including ease of manufacture (via stamping), low insertion force (preventing buckling or deformation of the fibers), fiber retention without the use of adhesives or epoxies, and reusability.
In spite of the foregoing achievements, however, the mass splicing of fibers in a reliable, quick and economic fashion remains a problem. For example, prior art hinged splice elements do not always bend along the same line on the splice element, and there is a high rejection rate during production. Without precise folding of the element, parallel to the fiber receiving grooves, fiber alignment and retention is affected since it results in inaccurate registration of the two halves of the splice element, and is especially critical when the two halves have complimentary V-grooves. It has also been found that ductile hinge elements, such as that disclosed in U.S. Pat. No. 4,824,197 (not prior art), require an annealing step after embossing in order to provide a hinge which will consistently survive a 180.degree. fold.
The sudden clamping transition near the fiber interface also causes deformation of the fiber resulting in more signal loss than if there were a more gradual clamping toward the interface.
Prior art optical splices also do not adequately address the optimum geometry for V-groove designs. For example, in the previously referred to FIG. 4 of U.S. Pat. No. 4,046,454, the V-grooves have obtuse angles, meaning that the four points of contact will not be completely symmetrical about the fiber. This may result in unnecessary transverse offset of the fibers, leading to greater splice loss. This is also true for hinged splice elements wherein a flat surface compresses the fiber into a 60.degree. V-groove. Since the flat surface is hinged to the grooved surface, and since the fiber is only partially embedded in the groove, the flat surface is not parallel to the groove-bearing surface when the splice element is in its closed, clamping state. See, e.g., U.S. Pat. No. 5,013,123 (this patent does not constitute prior art). Since these two surfaces are not parallel, the three lines or surfaces contacting the fiber will not be symmetrically positioned about the fiber, again adversely affecting transverse offset of the fiber end faces.
One final disadvantage relating to prior art optical splices concerns the use of a medium for matching the index of refraction of the two fibers. As mentioned above, reflective losses may be minimized by placing an index matching fluid or gel at the fiber interface. Oftentimes, however, this gel has bubbles, contaminants or other discontinuities which tend to migrate during the splice operation, and thereafter with temperature cycling. Such migration of the gel and microbubbles can detrimentally affect the splice quality. It would, therefore, be desirable and advantageous to devise an optical splice element which would obviate any problems associated with gel migration, as well as overcome the aforementioned limitations regarding a predictable hinge fold line, optimum V-groove geometry, and gradual clamping of the splice element.